1. Field of the Invention
The present invention relates to methods and structures for extremely quiet transformers which may be made within existing size and weight constraints.
2. Description of the Prior Art
Noises emanating from magnetic structures when excited by an electrical current are caused by the movement of the magnetic medium in response to each alternating cycle of the applied voltage. This movement is transmitted to the surrounding air and is defined as airborne noise. It is also transmitted to the mounting means where it is termed as structure borne noise. The excitation coils or windings that carry the alternating currents contribute little noise to the surroundings. The sound emanating from such magnetic structures exhibit a wide frequency band often exceeding the normal range of human hearing.
Magnetostriction, a phenomena that takes place in a magnetic media causing dimensional change and movement, is dependant upon level of magnetization. Magnetostriction is the major contributor to noise. The present invention minimizes the magnetostrictive forces in the magnetic paths so as to achieve a "quiet magnetic structure".
Several prior art patents discuss controls and limitation of transformer noise. These patents are described below.
U.S. Pat. No. 3,007,124 to Robinson discusses reduction of noise caused by vibrations in a magnetic structure core. The noise reductions are occasioned by the use of a laminated magnetic core structure where the laminations are held together by non-short-circuiting bands along the legs of the rectangular core. Further, the laminations are supported at the nodal points of one phase of the magnetic structure and are "lightly" clamped through a framework. Alternative methods of supporting the lower end of the transformer core are shown in the FIGS. 1-2, 3, and 4. It is described that the pinning required is to be inserted in the laminations and may be small because the laminations are held together by the bands.
U.S. Pat. No. 3,102,246 to Honey et al relates to a large power transformer located in suburban locations on electric power lines. A liquid dielectric is proposed in tubes with means to change the pressure in the liquid. The tubes serve to dampen the vibratory energy in the oil.
U.S. Pat. No. 3,125,735 to Twomey relies on specific mounting structure in an enclosure to limit the vibrations to the enclosure from a large electrical power distribution transformer.
U.S. Pat. No. 3,316,515 to Bock, et al discloses a magnetic structure separated by spacers such as 36, 38 and 40. Lock plates 42 and 44 are disposed along the length of each of the legs of the transformer adjacent to and contacting the outer laminations of each stack. Each stack of laminations has an opening through the stack so that a nut and bolt can be inserted between the elements of the stack and the supporting structure. The spacer is designed with a dimension which will decrease when the clamping force is applied thereto. The spacer does not change the gap spacing. The specific spacer design is shown in FIGS. 2 and 3.
U.S. Pat. No. 3,683,303 to Avano, et al encases the core and coil elements in a mixture of unsaturated polyester and a polyurethane. This relates to fluorescent lamps and to reduction or elimination of the transformer hum from the ballasts in fluorescent lights.
U.S. Pat. No. 3,815,067 to Koh produces a structure of a laminate in the shape of a U. In the space defined by the U Koh mounts a shock absorbent pad form and other laminates 5 and 7 which have surrounded by coil 6. The structure eliminates and reduces vibration and noise.
U.S. Pat. No. 4,047,138 to Steigerwald quiets the transformer by establishing radial air gaps at both ends of a ferrite magnetic core structure which includes rectangular outer members having aligned circular apertures. The patent relies on controlling the flux pattern and specifically the infringing flux pattern to reduce heat and vibrations.
U.S. Pat. No. 4,055,826 to Franz shows a high current reactor in which the current carrying windings are resiliently supported on a magnetic core. The windings are mounted on a series of springs.
U.S. Pat. No. 4,724,413 to Kataoka relies for noise suppression on sound-proof envelope 20 surrounding the primary winding of transformer 21. The sound-proof envelope is made of sound absorbing material such as silicone rubber or sound reflecting material such as metallic coil.
The article by Lin and Zook, TRANSFORMER CORE CONSTRUCTIONS, published in 1989 Coil Winding Proceedings, (Sep. 25-28, 1989) by International Coil Winding Association, Inc. pages 163-168 discusses available transformer core configurations.